The present relates generally to fiber optic devices, and more specifically to isolators and amplifiers suitable for use in a fiber optic communication system.
U.S. Pat. Nos. 4,329,017 and 4,479,697 to Kapany et al. disclose a number of fiber optic communication devices for performing a wide variety of monitoring, splitting, switching, and multiplexing functions. The devices are based on the property of a spherical reflecting surface that a point source of light slightly displaced from the center of curvature is imaged with minimal aberration at an optically conjugate point symmetrically located with respect to the center of curvature. The disclosures of the '017 and '697 patents are incorporated by reference.
U.S. Pat. No. 5,028,104 to Kokoshvili discloses a bypass switch suitable for use in a fiber optic LAN ring structure. The switch features a node self-test channel which is implemented by use of a looped fiber. Developments in recent years have presented the need for fiber optic isolators in local area networks. In brief, an isolator is like a check valve that lets light propagate in a downstream direction, but not in an upstream direction. It is sometimes very important that any light reflected from fiber connectors or other devices not be allowed to propagate in the reverse direction where such light might be mistaken for a signal or might interfere with other signals. Indeed, if a laser amplifier in the LAN were to be subjected to stray reflected light, it could become an oscillator, with potentially disastrous results.
Prior art fiber optic isolators typically collimate the light emerging from the upstream fiber's end, pass the light through a polarizer, a 45.degree. Faraday rotator, and an analyzer having its axis at a 45.degree. to the axis of the polarizer, and focus the light onto the downstream fiber's end. Clearly, the light emerging from the upstream fiber's end passes through the polarizer, is rotated by an amount that corresponds to the analyzer axis, and thus passes through the analyzer. Conversely, reflected light emerging from the downstream fiber's end passes through the analyzer, is rotated by the rotator, and blocked (attenuated) by the polarizer. A prior art isolated laser amplifier includes an upstream isolator as described above, a multiplexer which combines the signal radiation with pump radiation, a length of doped fiber in which the signal is amplified, and a downstream isolator.
A typical implementation of the Faraday rotator in these devices is a yttrium iron garnet (YIG) crystal surrounded by a solenoid. Typically the solenoid is energized to provide sufficient magnetic field to saturate the crystal, and the crystal thickness is selected to provide 45.degree. rotation. Unfortunately, the YIG crystals that are used for the isolators are expensive.